The invention provides 4Cr3MoWVNi (B3R) hot work die steel with high strength and toughness and resistance to tempering, and a preparation process thereof. Raw materials for preparing the B3R hot work die steel comprise, by weight, C 0.35 to 0.42%, Si 0.15 to 0.35%, Mn 0.3 to 0.5%, Cr 2.50 to 3.50%, Mo 1.00 to 1.50%, W 0.50 to 0.80%, V 0.50 to 0.80%, Ni 1.45 to 1.85%, S 0.003 to 0.010% and P 0.008 to 0.018%, with the balance being Fe. The preparation process comprises the following steps: smelting at intermediate frequency--remelting (refining) of electroslag-diffusion hydrogen of electroslagingots, annealing treatment of electroslag ingots--forging--annealing of forging stocks--rough machining--vacuum heat treatment, and secondary tempering so as to obtain the novel product of die steel. The hot work die steel prepared in the invention has high Rockwell hardness, excellent impact toughness and tempering (thermal fatigue) resistance, and the advantages of optimized collocation of components, pure steel quality, a uniform structure, high strength, strong toughness, good tempering resistance, etc.